Distribution and synaptology of glossopharyngeal afferent nerve terminals in the nucleus of the solitary tract of the hamster.

The distribution and synaptology of the afferent fibers of the glossopharyngeal nerve (IXN) in the hamster were studied by using horseradish peroxidase (HRP) histochemistry visualized with light and electron microscopy. Crystals of HRP were applied to the trunk of IXN in the vicinity of the petrosal ganglion. The densest IXN afferent label was distributed within the nucleus of the solitary tract (nst), just caudal to but overlapping with the area of termination of the facial nerve. Labeled IXN fibers extended rostrally to the principal trigeminal nucleus and caudally to the cervical spinal cord. There was significant labeling within the spinal trigeminal complex; the area postrema and the medullary reticular formation contained some labeled fibers. Ultrastructurally, the synaptic arrangements of anterogradely labeled IXN fibers were examined in the nst. Quantitative measures were taken of the area, maximum diameter, perimeter, and vesicles of labeled endings and the length of their synaptic junctions with dendritic processes. These endings were compared to comparable endings in control material and to published descriptions of VIIth nerve afferent terminals in the hamster nst. The synaptic relations of IXN afferent endings were predominantly with dendritic spines and shafts. The majority (86.6%) of IXN afferent endings were with dendritic processes that were not in apparent contact with other, unlabeled processes. Only 13.4% of IXN synaptic relationships were with dendritic processes that were also contacted by unlabeled vesicle-containing processes. This is in contrast to 31.2% of facial nerve afferent endings in the nst which make synaptic contact with such processes. There were more direct synaptic contacts between facial endings and unlabeled vesicle-containing processes (26.1%) than between IXN endings and unlabeled vesicle-containing processes (1.3%). Thus, unlike the glomerular-like endings of the gustatory fibers of the VIIth nerve, less complex relations appeared to characterize IXN synapses in the nst. These differences were related to the differential physiology of gustatory fibers in the VIIth nerve and IXN.